Laying Out Bridge Scour Cross Sections

Using Reference Lines to Access Transect Hydraulics in HEC-RAS

To extract the data from HEC-RAS required to perform a bridge scour analysis, you will have to define Reference lines for the 3 to 9 cross sections the model requires.

Reference Lines are a relatively new feature in HEC-RAS that allow users to query and summary hydraulic parameters (averages and maximums) from a specified transect.  They are different than "Profile Lines" in a several important ways. 

Most importantly, reference lines are part of the geometry and HEC-RAS writes summary results to them during the simulation.  The 2D results HEC-RAS writes to these reference lines intentionally includes the cross-section averaged summary results that HEC-18 requires for bridge scour analysis (e.g. hydraulic depth, average velocity, max velocity, etc...).  Profile lines have some utility in Bridge Scour analyses, but most of the 2D HEC-RAS results you will use will come from Reference Lines.

Laying out Reference Lines for 2D Bridge Scour Analyses

The FHWA HEC-18 analysis requires three cross sections, and each of these cross sections are divided into three sub-sections (channel, left overbank, right overbank). 
You will define:

• An Approach Cross Section: This cross section is upstream of the bridge where the river is at its widest in flood flows - occupying the maximum floodplain extent. 
• A Contraction Cross Section: This cross section follows the center line of the bridge and represents the narrowest flow constriction.
• An Upstream Pier Cross Section:  Most of the scour calculations just use the approach and contraction cross sections, but pier scour requires summary data from a cross section just upstream of the bridge.  This is sometimes confused with the Approach cross section (and the same cross section was often used for both in 1D analyses) but is distinct.  The upstream face cross section is usually about one pier length (usually the bridge width in HEC-RAS) upstream of the upstream face of the bridge and is used to compute a maximum channel velocity and depth entering the bridge.  The toolbox uses this 

The following example illustrates reference line placement on an idealized model.
Expand the link below for some more realistic examples in different river settings.

The figure below is an example of the nine reference lines laid out for a bridge with a geologic contraction, requiring the approach cross sections farther upstream

In the example below, the Approach cross section is closer to the bridge than it would be in most cases, but careful inspection of the flow paths indicates that the local geology and complicated flow paths put locate the transect of maximum expansion at the approach XS.

(Note: it can sometimes be challenging to capture the transect of maximum expansion in the left and right overbank while connecting to an orthogonal channel cross section.  But it is essential to keep your cross sections perpendicular to flow, especially in the channel because an angled cross section will over-represent the width of the channel and, therefore, the contraction.) 

Tips For Defining your Reference Lines

Defining the reference lines requires engineering judgement and introduces subjectivity into these analyses.

1.  Align Reference Lines with Cell Faces: Reference lines use more sophisticated logic than profile lines to project the fluxes (e.g. flow) and vectors (e.g. velocity) from cell faces to a summary transect.  But it is still worth recognizing that HEC-RAS only knows flux and vector properties at cell faces.  Therefore, the more that reference lines align with cell faces, the more accurate the summary data in the reference line will be.  Enforcing a break line (or an arc in 2025) is good practice to make align cell faces with a break line.

2. Digitize from Left to Right: Downstream flow and velocity will be positive if you digitize these XS from left to right looking downstream.

3. Begin Contraction XSs Where Your Bridge Begins: Begin and end your You will find extracting bridge data and populating the Hydraulic Toolbox easier if your contraction reference lines start at the same location as your bridge deck.  This will keep the stationing of the hydraulic cross section consistent with the bridge and piers. (Note: HEC recommends starting your bridge very close (within 1 or two cells) of the opening like pictured below)

4. "Channel" is Generally Toe-to-Toe not Bank-to-Bank: The transporting channel is generally narrower than the hydraulic channel.  Rivers tend to transport bedload on the channel bed, between the bank toes.  Therefore, draw your channel reference line between the bank toes, not the top of the banks (see image below).

5. Does the Bridge Backwater Cause a Clear Water Condition?: Bridges create backwater conditions.  If your approach cross section is upstream of the bridge's backwater effect it may compute a live bed condition that does not make it all the way to the bridge (i.e. the velocity just upstream of the structure drops below the critical velocity).

To define a reference line, start editing the geometry.

Select the Reference Line node of the tree and digitize each cross section component independently (e.g. make the Approach Channel and Approach ROB adjacent but separate cross sections).  Digitize these reference lines from left to right looking downstream.  (Cross sections digitized in the opposite direction will work, but will report negative signed results).

Tips For Selecting Your Approach XS Location

1. Run the Model and Examine the Flow Paths: It is useful to simulate the design flow before you define your reference lines to harvest results.  The inundation extents and trace lines can help you select the approach cross section location and lay out the cross sections perpendicular to flow.
2. May Be Farther Upstream Than You Expect: Conversations with Federal Highway subject matter experts suggest that many HEC-RAS models place the approach cross section too close to the bridge.

Channels are seperated from "banks" by different criteria for bridge scour